Refuse pyrolysis has assumed increasing importance in recent years as a possible alternative to refuse incineration. Pyrolysis of refuse yields gas that is useful as an energy source or as raw material; besides, suitable by-product gas purification makes such pyrolysis more effective and permits economical operation of even those refuse pyrolysis plants which have a relatively small capacity.
A refuse pyrolysis furnace ordinarily has several zones, e.g. a drying zone, a conversion zone and a gas generation zone. In one particular operation the furnace has the same wall temperature, e.g. about 750.degree. C., in all zones. Refuse in the drying zone, however, attains only a considerably-lower temperature, e.g. 250.degree. C., since drying requires a great deal of heat. Expelled water remains in generated gas in vapor form. Refuse in the conversion zone attains a temperature of, e.g., 550.degree. C. and a temperature of 750.degree. C. is attained in the gas generation zone. Meltable components are melted down and preliminary sintering is effected in the conversion zone, wherein degasification is initiated. All substances which are gasifiable (at degasification temperature) are gasified in the gas generation zone.
Exit gas from the furnace ordinarily contains, in addition to much water (often about 50%), a series of combustible substances, such as CO, CH.sub.4, H.sub.2, C.sub.2 H.sub.4, C.sub.2 H.sub.6, C.sub.6 H.sub.6, and additional harmful or noxious substances, such as CO.sub.2, HF, HCl, SO.sub.2, H.sub.2 S, cyanides, NO.sub.X (e.g. NO, N.sub.2 O, NO.sub.2, N.sub.2 O.sub.4), phenol, benzanthrene, chrysene, naphthalene, naphthenes, furol, acetone, methanol, acetaldehyde and ammonia. The percentages vary greatly, depending upon the charged refuse and pyrolysis parameters, such as temperature. Some of the noted noxious substances, e.g. unsaturated hydrocarbons, form tar-like contaminants in the gas and thus create particular difficulty during gas purification. Unsaturated hydrocarbons tend to polymerize, and such polymerization may be favored by pyrolysis conditions or during subsequent processing.
Water is used to purify the gas from the furnace, also referred to as pyrolysis gas, which has an exit temperature in excess of 600.degree. C., and ordinarily at least 750.degree. C.
As the pyrolysis gas contains a large proportion, perhaps as much as 50%, of water, a large volume of water is obtained during its purification. Some or all of the harmful or noxious substances, as well as useful components of the pyrolysis gas, are likely to become admixed with or part of one or more aqueous or oil phases or a water-oil emulsion. Separation of still-usable constituents of the pyrolysis gas from water is not difficult. Additional difficulties result from the need to purify water (containing harmful or noxious substances) before it can be discharged as waste water. That requires considerable technical effort.